The clay courts of Roland Garros may mystify American men, who've won the French Open just four times since 1956, but they're no mystery to Dr. Stuart Miller. The chief technical wizard for the International Tennis Federation has analyzed the earthy mix of clay surfaces (a combination of rocks, pebbles, clay, mud and brick) and has developed an interesting explanation for why big-serving Americans flounder at the only Slam played on clay.

Stuart bases his analysis on something called frictional coefficients, numbers that indicate the resistance between court surface and ball. Working with those numbers-which governing bodies use to pick balls for tourneys-Stuart developed a computer simulation that measures and shows a ball's rebound speed, or "surface pace rating." Clay, the slowest surface, has a rating of about 20; the hardcourt acrylic surface at the U.S. Open is in the upper 30s; Wimbledon's grass courts are fastest, in the low 40s.

Translated into real numbers, the simulation shows that a neutral-spin, 120 mph serve on clay (which absorbs a serve's power) takes .077 seconds longer to reach the baseline than a serve hit on acrylic, and .101 seconds longer than one hit on grass. Players are instinctively aware of this, Miller says, so they "don't waste energy trying to kill the ball." In fact, the average speed for the 20 top servers at the French (134 mph) is 4 mph slower than at Flushing Meadow and Wimbledon.

Serves on clay also bounce higher than on other surfaces. The same 120 mph serve rebounds 9.84 inches higher on clay than on acrylic, and 17.32 inches higher than on grass. Slower pace and extra bounce give receivers more time to set up, which results in fewer aces. In 2005, 1,435 aces were hit at Roland Garros, versus 1,648 at the U.S. Open and 2,080 at Wimbledon.

So how does this explain the record of U.S. men in France? Simple: Americans generally depend on big serves and short rallies, but clay rewards patient, well-conditioned players such as defending champ Rafael Nadal. So it's no surprise that U.S. women, who don't rely as much on power, have historically fared better than their male counterparts. And it's even less of a shock that eight of the past nine men to win the French are from countries that love their siestas.